a) Field of the Invention
The present invention relates to photolithography for semiconductor fine patterning processes, and more particularly to a method of forming a fine resist pattern on a conductive thin film which reflects exposure light.
b) Description of the Related Art
For high integration of semiconductor integrated circuit devices, not only fine patterns are required but also high resolution of photolithography is necessary. In order to improve the resolution, the wavelength of exposure light tends to become short and the numerical aperture of a lens system in a stepper tends to become large. This tendency narrows a pitch between interference stripes on an exposure surface.
In addition to the above problem, there are other problems associated with device fine pattern formation. One of these problems resides in that a resist pattern is deformed by the light reflected from the surface of an underlie layer on the substrate, and the other resides in that the depth of focus becomes insufficient.
The light reflection problem occurs when a resist pattern is formed on a conductive film having a high reflectivity relative to exposure light. Exposure light incident upon a resist film is reflected at the surface of the conductive film. Standing waves generated by interference between incident light and reflected light forms a repetitive exposure intensity distribution in the thickness direction of a resist film, and the cross section of the developed resist film is corrugated. If the surface of the underlie layer on the substrate is irregular, scattering and converging of reflected lights occur depending on the irregular shape. Convergence of reflected lights in particular causes halation.
Exposure is performed nowadays by using excimer light (KrF 248 nm). In this case, reflection at silicon itself of the substrate is also strong and similar light reflection problem occurs.
For the light reflection problem, a method of laminating an antireflection film on a conductive film or a use of dye-containing resist has been tried. See J. J. Ferrari, "Comparison of Deep UV Reflection Control Method for Interconnect Layers", Proceedings of the SPIE, Vol. 2197, pp. 660-668, 1994, and Willard Conley, "Improved Reflectivity Control of APEX-E Positive Tone Deep UV Photoresist", Proceedings of the SPIE, Vol. 2195, pp.461-477, 1994. Both the methods relieve the influence of reflected light by attenuating exposure light reflected from the conductive film surface and returned to the resist film to lower the intensity of light.
The use of an antireflection film requires an additional process of forming the antireflection film on a conductive film. In contrast, use of dye-containing resist does not increase the number of processes because it is sufficient if only the resist is replaced.
In forming a high precision pattern on an irregular surface, a deeper depth of focus is required. However, the numerical aperture of a lens system in a stepper is increasing in order to improve the resolution. A large numerical aperture makes the depth of focus shallow and makes it insufficient. As a method of realizing a deeper depth of focus, an exposure method (modified exposure method) has been proposed recently, which uses a modified illumination system in which principal illumination light is inclined relative to the principal optical axis. See Keiichiro Tounai, "Optimization of Modified Illumination for 0.25 .mu.m Resist Patterning", Proceedings of the SPIE, Vol. 2197, pp. 31-41, 1994.
With a normal illumination method (a general exposure method without a modified illumination system in which principal illumination light is in parallel with the optical axis), and 0th order light is incident upon a substrate in the vertical direction. Light fluxes of +1st order light and -1st order light diverge on both sides of the 0th order light at an equal angle. In order to focus an image, three light fluxes including the 0th, +1st, and -1st order light fluxes are required to be converged. With the modified illumination method, 0th order light is incident in the oblique direction. In order to focus an image, 0th and +1st order light fluxes or 0th and -1st order light fluxes are required to be converged. One light flux among the three light beams can be omitted. Therefore, an angle formed by two focusing light fluxes at a substrate becomes small (about a half) and therefore the depth of focus can be made deeper.
Use of an antireflection film is effective for reducing reflected light. However, this method is not effective for the problem of an insufficient depth of focus, and increases the number of processes.
Use of dye-containing resist can relieve the influence of reflection from an underlie film. However, this method cannot solve the problem of an insufficient depth of focus. According to the experiments made by the present inventors, dye-containing resist lowers an image forming ability.
Exposure using a modified illumination system can make the depth of focus deeper. However, this method cannot solve the problem of deformation of a pattern shape by the influence of reflected light, especially halation.